Radiator drain tap thread

[Roger's E Type]

Hi all,
A brass replacement radiator drain tap has been fitted to my Series 1 4.2 2 + 2. It is the wrong thread and therefore the tap has a stripped thread. I think the female thread in the radiator is okay. Does anyone know what the thread size is? I need to clean the female thread before fitting the correct tap.
Cheers, Roger

[Heuer]

I tapped a new thread in my Radtec 'Marston' radiator as they thoughtfully used a metric one 1/2" UNC I seem to recall.

[Moeregaard]

It's been a looooong time since I've seen an original radiator, but I suspect the factory thread was either 1/8" or 1/4" BSP (British Straight Pipe), which has one extra thread per inch over the U.S. pipe-thread system. I mention this because I've seen a number of occasions where a US NPT (National Pipe Tapered) has been forced into a hole tapped BSP. Those of you who have cars that came from the U.S. are likely to encounter this problem.

[steve3.8]

Roger ,

It's ?" BSP originally , same in the engine block .

Upgrade ? ,

I bought a new drain tap which turned out to be poor quality so i fitted a modern ball tap into the bottom of my rad ,it's out of sight so it's not spoiling anything , it does'nt leak / weep /drip etc , the 8mm bore drains the coolant out stair rod staight and fast , i also drilled a small hole in the lever so i could fit locking wire .
Similar item here

http://www.ebay.co.uk/itm/Brass-Chrome- ... 1e90d8d4fd

[Heuer]

You need to buy the tap first and then figure out what the thread is. BSP would have been my first thought but I am sure (I just checked) I used UNC because I only own UNC/UNF/Metric taps.

[Roger's E Type]

Thanks everyone who replied. Turns out that the threaded hole in this radiator was 1/4"BSP. Thanks Steve too for the tip on the ball valve - I was able to get one from a local wooden boat builder! Now fitted, and holding water!
Cheers, Roger